1. Field of the Invention
The invention relates generally to electrical measuring equipment and methods. In particular, the invention relates to a high-frequency probe used for mapping resistivity and other electrical characteristics in a sample with resolution of substantially less than a millimeter.
2. Background Art
There is much interest in developing a microwave microscope that uses microwave radiation in the gigahertz range to measure one or more electrical characteristics of a sample and, by scanning the probe over the sample surface, to image the spatial variation of such characteristics. Such a microwave microscope would be very useful in the semiconductor integrated circuit industry for mapping resistance or dielectric constant over the wafer, particularly during its fabrication since a microwave measurement is non-destructive. The gigahertz measurement frequency corresponds to the important frequencies utilized in semiconductor devices. However, for integrated circuits, the imaging resolution must be on the order of no less than a few microns since feature sizes are being pushed to much less. However, microwave wavelengths and waveguide dimensions are in the range of centimeters to millimeters, far greater than the desired resolution.
Several proposals have been made for microwave probes that have a spatial resolution much less than the wavelength of the radiation being used. However, they all seem to depend upon a resonant structure. For example, Xiang et al. in U.S. Pat. No. 5,821,410 describe a sharpened probe tip extending through an aperture in a resonant xcex/4 cavity and projecting toward the sample under test. Such a cavity is resonant over only a narrow bandwidth band so that measurements at significantly different frequencies require multiple dedicated probes. However, it is desired that the microwave microscope be tunable over a substantial bandwidth in order to determine the frequency dependence of the material characteristics. When the narrow projecting probe is being scanned close to sample surface, it has the further drawback of being prone to strike the uneven sample surface and being permanently damaged.
Anlage et al. in U.S. Pat. No. 5,900,618 disclose a somewhat similar microwave microscope, which apparently has a wider bandwidth of operation, but it still relies upon resonance conditions in a microwave coaxial cable. It is difficult to make a coaxial cable having a diameter of the outer conductor of less than the 450 xcexcm minimum value of Anlage et al. without losing system senstivity.
A further disadvantage of a resonant structure is its needs to have a dimension at least a quarter of the wavelength of the probing RF or microwave radiation. Typical sensors operating in the gigahertz range have resonant cavities of 1 to 3 cm3 or coaxial cables 2 to 4 cm long. Such large sizes even for 10 GHz radiation makes the probe large, heavy, and thus slow to scan over a sample at a high sampling rate desired for imaging a relatively large area.
Davidov et al. in U.S. Pat. No. 5,781,018 disclose a microwave probe having a narrowly resonantly sized slit formed in the end of the microwave waveguide. While the waveguide itself is not resonant, the slit size is constrained to operation at one frequency, and the sensitivity of this system decreases rapidly as the size of the slit (and hence the resolution) is decreased.
Somewhat similar measurements can be made using a scanning capacitor measurement apparatus with a small tip electrode and the sample acting as the other electrode, such as disclosed by Williams et al. in U.S. Pat. No. 5,523,700, by Slinkman et al. in U.S. Pat. No. 5,065,103, and by Matey in U.S. Pat. No. 5,581,616 and reissued U.S. Pat. No. Re. 32,457. Calculations relate the measured capacitance over some measurement parameter such as DC voltage with electrical characteristics of the material. While these systems can be used to measure the complex impedance between tip and ground, when the sample constitutes the second electrode in opposition to the small tip electrode in the capacitance measurement, the sensed area extends far from the probe electrode, and it is difficult to relate the measured impedance to the dielectric constant and resistivity of the material.
A microwave microscope is scanned over a sample surface for imaging electrical characteristics of the sample and uses non-resonant probe and circuitry allowing sample characterization over a wide frequency range extending, for example, from 10 MHz to 50 GHz.
The probe preferably includes an outer electrode coated onto a conically shaped dielectric disk and having a central aperture in which the inner electrode is disposed. The inner electrode may have a sharpened tip and be disposed in a bore extending through the dielectric disk. The outer electrode may be grounded, and biasing and measuring circuitry is connected to the inner electrode.
The circuitry may include a negative feedback amplifier with low input impedance to measure the current between the electrodes and configured to couple the RF or microwave drive signal to the inner electrode. A signal processor receiveing the output of the amplifier may detect the in-phase and out-of-phase components (or magnitude and phase) of the amplifier output. Alternatively, the amplifier may have a high input impedance and thus measure the potential across the electrodes.
Optionally a guard electrode is disposed in the dielectric disk between the inner and outer electrodes. In this embodiment, the drive signal may be connected directly to the guard electrode and is coupled to the tip through the capacitance between these two electrodes.
The probe and measuring circuitry are non-resonant and can be driven at selected frequencies within a wide frequency range, for example, 10 MHz to 50 GHz or above. Thereby multi-frequency measurements benefit from a tunable drive source.